1. Field of the Invention
This invention relates to a programmable control device, and more particularly, to a device whose control unit is connected to a general-purpose bus to externally control a controlled unit.
2. Description of the Related Art
Heretofore, two types of devices each of which incorporates a control function have been known, i.e., a type in which the operational function of the device is controlled by hardware logic, and a type in which control programs and a CPU (central processing unit) are incorporated within the device, and the CPU controls the operational function of the device according to the control programs. The latter type device includes an arrangement for externally inputting a control program for a computer or the like, whereby the device can have an entirely different function by use of the input control program although the same hardware is used.
FIG. 3 is a block diagram showing an example of the configuration of a conventional programmable control device.
In FIG. 3, a CPU 201 controls the entire device by interpreting and executing programs stored in a ROM (read-only memory) 202. A RAM (random access memory 203 functions as a work memory to be used when the CPU 201 performs its operation. The ROM 202 and the RAM 203 are selected by addresses output from the CPU 201 to an address bus S201. When the ROM 202 is selected, the contents of the memory are output to the data bus S202. When the RAM 203 is selected, data are input and output via the data bus S202. A first I/O unit (input/output interface unit) 204 is selected by an I/O address output from the CPU 201. The first I/O unit 204 controls an information display unit 205 by transmitting control data output from the CPU 201 to the information display unit 205. A second I/O unit 208 controls an information output unit 207 by transmitting control data output from the CPU 201 to the information output unit 207, in the same manner as the first I/O unit 204. Thus, information is output to a mechanism control unit (not shown) provided within the device, or outside of the apparatus. A third I/O unit 208 outputs control information and other information from an information input unit 209 to the CPU 201. A fourth I/O unit 210 outputs control information and other information detected by an information detection unit 211 to the CPU 201.
However, even a device which can be controlled by inputting a control program from the outside (a programmable control device) has the problem that it becomes obsolete within a very short time in accordance with technical progress, since the same incorporated hardware is used.
That is, while the performance of the CPU 201, and the capacity, processing speed and the like of the memory, such as the RAM 20S, advance day by day in accordance with progress in the current semiconductor technology, a device having hardware that is fixed cannot accommodate components developed by such new technology in its structure. Accordingly, a process of developing a new device which accommodates components developed by the new technology is in many cases adopted. As a result, the lifetime of a product shipped to the market is shortened by being replaced by a next-generation product, causing waste of available resources.